The loss or the overexpression of proteoglycans constitutes a physiopathological biomarker of several cartilage degenerative pathologies such as osteoarthritis, arthritis, intervertebral disk diseases as well as aneurysm or aortic dissection. Overexpression of proteoglycans is also encountered in chondrosarcoma, a malignant tumor composed of cartilage-producing cells.
Due to the ageing population, cartilage degenerative pathologies such as osteoarthritis, set serious public health issues in developed countries. Until now, the therapeutic management is only symptomatic. More than 14 million of prescriptions per year in France aim to reduce and control symptoms such as pain and functional discomfort.
Concerning osteoarthritis, there is consensus regarding the necessity of an early diagnosis, based on the perspective of disease modifying drugs for osteoarthritis (DMOAD). However, at an early stage, clinicians lack of objective, reliable and sensitive criteria for evaluating the progression of the disease.
The current gold standard method, based on the assessment of joint-space narrowing by X-ray techniques, is limited in terms of precision and sensitivity. Ding et al. (Nat. Clin. Pract. Rheumatol. 2008, 4, 4-5) report that knee cartilage volume is reduced more than 10% until the first radiographic abnormalities were detected.
MRI techniques and dosages with relevant biochemical markers have also been evaluated. These methods however lack of specificity and/or sensitivity, and are unable to detect slight biochemical modifications associated with osteoarthritis (Ding et al., Curr. Opin. Rheumatol. 2013, 25, 125-135). In another hand, current available radiotracers only explore bone remodeling metabolism (e.g. 99mTc-hydroxymethylene diphosphonate, 99mTc-HMDP) or inflammatory processes (e.g [18F]-2-fluoro-2-deoxyglucose, [18F]FDG).
With these conventional radiotracers, imaging techniques such as Single-Photon Emission Computed Tomography (SPECT, or less commonly, SPET) or Positron Emission Tomography (PET), eventually combined with X-ray Computed Tomography (SPECT-CT or PET-CT), only provide indirect information of the cartilage degradation during the late stages of the disease, namely the inflammation and alteration of the subchondral bone steps (Tremoda et al., EJNMMI Res. 2011, 1, 11).
Cancerous cartilage diseases (i.e. chondrosarcoma) represent 25% of the total primary malignant bone tumors diagnosed in humans. The tumor develops from chondrocytes and is thus rich in proteoglycans (PG). It can be found in multiple histological forms (grades 1, 2 or 3) which differ in terms of treatment and prognosis. The 5-year survival is 80% for chondrosarcoma of grade 1, 50% for chondrosacoma of grade 2, and only 20% for chondrosarcoma of grade 3 (Angelini et al., J. Surg. Oncol. 2012, 106, 929-937).
Yet, there is a real expectation of clinicians in this domain, taking into account the low sensitivity or/and specificity of conventional imaging techniques such as radiography, scanner and MRI. One the one hand, CT and MRI define morphology, but are unable to distinguish postoperative or post-chemotherapeutic residual lesions due to altered tissue planes, edema and fibrosis. (Soldatos et al., J. Comput. Assist. Tomo., 2011, 35, 504-511; Logie et al., Semin Musculoskelet. Radiol. 2013, 17, 101-115). One the other hand, radiotracers currently available such as 201Tl, 99mTc-MIBI, 99mTc-Tetrofosmin, 99mTc-DMSA(V) and 18F-FDG, provide indirect evaluations of the pathology, and have demonstrated their limitations for imaging chondrosarcoma with low cellularity and vascularity (Brenner et al., Eur. J. Nucl. Med. Mol. Imaging 2004, 31, 189-195; Murata et al., Ann. Nucl. Med. 2008, 22, 221-224; Douis et al., Skeletal Radiol. 2013, 42, 611). Therefore, there is an urgent need for markers that characterize biologic phenotypic features of the tumor to guide clinical decision making (Riedel et al., Curr. Treat. Options Oncol. 2009, 10, 94-106).
Proteoglycans (PG) constitute with collagen the essential biochemical components of the extracellular matrix (ECM) of connective tissues as cartilage. The proteoglycan matrix of chondrosarcoma is composed mainly of aggrecan-type proteoglycans. Aggrecan is a macromolecule made up of a core protein substituted with covalently linked glycosaminoglycan (GAG) chains.
According to literature, there are similarities between articular cartilage and intervertebral discs (IVD). Healthy IVD are comprised of three different areas: Annulus Fibrosus (AF) which surrounds the Nucleus Pulposus (NP), and the vertebral end plates embedding the AF and NP (Urban et al., Arthritis Research and therapy 2003, 5, 120-130). The few cells found in IVD are similar to articular chondrocytes, while the ECM of IVD and hyaline cartilage share the same main components: collagen and PG. As for articular cartilage, most of the PG found in IVD are aggrecans. Remodeling of both cells and PG of NP is considered to be relevant biomarkers of IVD degeneration at earlier stages of pathologies, associated with lumbar pain. Indeed, during IVD degeneration, the NP is the first component affected with i) an alteration and remodeling of the PG content leading to dehydration and disorganization of the ECM, and ii) a decrease of cell density (Lyons et al., Biochim. Biophys. Acta 1981, 673, 443-453; Antoniou et al., J. Clin. Invest. 1996, 98, 996-1003; Roughley et al., Spine 2004, 29, 2691-2699; Adams et al., Spine 2006, 31, 2151-2161; Clouet et al., Spine 2009, 76, 614-618).
However, imaging techniques conventionally used for the evaluation of this disease (X-ray, MRI and CT) also provide anatomic information (calcifications, radial cracks, disc pinching . . . ) only at very late stage (Sether et al., Radiology 1990, 177, 385-388; Pfirrmann et al., Spine 2001, 26, 1873-1878; Finch et al., Nat. Clin. Pract. Rheumatol. 2006, 2, 554-561).
Other diseases, such as aneurysms and aortic dissections are characterized by the presence of mucoid degeneration and alcianophil areas rich in GAG (Houard et al., Pathology 2007, 212, 20-28; Sarda-Mantel et al., Arterioscler. Thromb. Vasc. Biol. 2006, 26, 2153-2159; Roccabianca et al., Biomech. Model. Mechanobiol. 2014, 13, 13-25).
For above pathologies, measuring PG disorders (loss or overexpression) could permit early diagnosis and also assessment of extent, longitudinal follow-up of treated patients and evaluation of new therapies.
Because of the high sulfate and carboxyl group content of their GAG moieties, PG have strong negative charges that have been shown to interact with the positively charged quaternary ammonium function (QA) (Gibbs-Strauss et al., Molecular Imaging, 2010, 9, 128; Maurizis et al., Biochem. Pharmacol. 1992, 44, 1927; Sidney Yu et al., Int. J. Rad. App. Instrum. B, 1989, 16, 255)
French patent application FR 2 795 412 A1 discloses compounds with a strong affinity for cartilage tissues containing quaternary ammonium function of the following formula:
wherein M represents a molecule useful in the treatment or diagnosis of diseases caused by cartilage damage, notably as radiotracer. More specifically, this document reports 99mTc-NTP 15-5 complex having the following formula:
which exhibits a high uptake in the PG-rich tissues after intravenous injection (i.v.). Since this statement has been reinforced by articular cartilage imaging (Ollier et al., J. Nucl. Med. 2001, 42, 141-145; Miot-Noirault et al., Eur. J. Nucl. Med. Mol. Imaging 2007, 34, 1280-1290; Miot-Noirault et al., Molecular Imaging 2008, 7, 263-271) and chondrosarcoma imaging studies (Miot-Noirault et al., J. Nucl. Med. 2009, 50, 1541-1547; Peyrode et al., Sarcoma, 2011, Article ID691608, 8 pages; Miot-Noirault et al., EJNMMI Res. 2013, 3, 40).
SPECT imaging is limited in terms of sensitivity and quantitative performances and thus is little suitable for early diagnosis and/or follow-up of pathologies involving cartilage degeneration such as osteoarthritis, arthritis or chondrosarcoma.
PET imaging appears to be more appropriate to solve the above-mentioned issues. Radiotracers for PET imaging require radioisotope elements that emit positrons, whose annihilate to produce two photons with opposite direction. Radioisotope elements used for radiotracers within PET include notably 124I, 68Ga, 64Cu, 18F, 15O, 13N, or 11C (Serdons, K, Verbruggen, A, Bormans, G. M. Developing new molecular imaging probes for PET. Methods 2009, 48, 104-111).
Such radioactive compounds could be used to explore physiological and pathological processes within the limits of several requirements, notably in terms of specific binding to target tissues, thermodynamic stability, in vivo kinetic inertness, adequate elimination (such as urinary clearance), low non-specific uptake in non-target tissues, availability, toxicity and appropriate radioactive isotope so as to ensure good target to non-target contrast.
Two kinds of radiotracers are currently known:
(1) chemical compounds in which one or more atoms have been replaced by a non-metallic radioelement (i.e. 124I, 18F, 15O, 13N, 11C . . . ), like 18F-FDG,
(2) bifunctional chelators (BFC) referred herein to compounds made up of a chelating ligand for chelation of metallic radioelement (i.e. 68Ga, 64Cu . . . ) through coordinate covalent bonds, linked to a functional group that can be used for attachment to a targeted molecule (e.g. antibodies, peptides, proteins). One of the BFC's advantages is that the chemistry of conjugation can preserve the structural integrity of the targeted molecule as no extremes of temperature or pH are required.
The estimation of the overall complex charge for new radiolabelled BFC, which is a key parameter when developing PG-targeting radiotracers, must be assessed case by case.
Even if complex stability forecasting and in vitro complex stability measurements give an accurate starting point, they can be questioned by in vivo experiments (I. Sin et al., Bioorg. Med. Chem. 2014, 22, 2553-2562).
If numerous studies deal with radiotracer for PET imaging, none has been reported on targeting cartilage PG specifically. As shown previously, there is a need for a non-invasive method evaluating integrity and functionality of cartilage and allowing an early diagnosis, a longitudinal follow-up and evaluation of therapy response for diseases associated with a deregulation of the proteoglycan concentration such as osteoarthritis, arthritis, chondrosarcoma, IVD degeneration or aneurysm and aortic dissection.
There is therefore a need for diagnostic agents that are able to target proteoglycans and are suitable for use within PET imaging.